Builders seeking performance beyond the realm of bolt-on supercharger and turbocharger kits— exceeding about 10 pounds of boost, or so—likely need to consider the construction of a custom engine assembly designed specifically for forced induction. In the simplest terms, that means replacing the factory cast rotating parts with premium, forged components; ensuring greater head-clamping power and optimizing the compression ratio.
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Even vehicles with bolt-on forced-induction systems benefit from a purpose-built engine that supports the power adder, as the engine will likely offer greater durability, resistance to detonation, and more overall power. Although any engine buildup is not regarded as inexpensive by most builders or enthusiasts, there are methods to simplify the process and keep the overall cost to a minimum.
The seemingly easiest and leastexpensive option is simply upgrading the vehicle’s existing engine with a forged rotating assembly and boostcompatible, lower-compression pistons. Of course, choosing this option or a more extensive engine buildup requires the removal and disassembly of the original engine. In other words, with the heavy lifting required to remove the engine, making the investment in the engine is better justified in the long run.
It’s important to keep in mind that while GM’s LS engines are commendably robust, durable, and reliable, only the recent LS9 and LSA versions were designed explicitly for supercharging—and all of the engines were tested and validated to perform within carefully engineered parameters. That means, for example, the rods and pistons of the LS7 engine are designed to deliver the 505 rated horsepower within the stated RPM range with a small percentage buffer, but strong as that engine may be, its components were not validated for forced induction.
Building in strength and durability is paramount in the engine’s overall success and longevity. The tremendous power gain delivered by the supercharger or turbocharger lessens the effective differences that lower-mass (lighter) components offer on a naturally aspirated engine. That means the instinct to use, for example, lighter-weight pistons to maximize performance isn’t necessarily the correct one, as a heavier forged-alloy piston may slightly increase friction, but ultimately prove stronger under maximum boost. And at, say, 20 pounds of boost, the marginal weight difference won’t be noticed. In other words, using the strongest rotating parts in addition to a strong cylinder block and premium fasteners is worth the few RPM they may sacrifice in the long run in order to ensure optimal cylinder pressure.
Cylinder Block
Unless you are planning to use the original engine from your vehicle as the starting point, there is almost an unlimited number of options when it comes to selecting an appropriate cylinder block to use as the new engine’s foundation.
The GM Performance Parts LSX cylinder block was designed with forced induction in mind. Its cast-iron construction is not only strong, but makes it very economical; less than $2,000 from most retailers. It is offered in two deck heights: standard (PN 19166454) and tall-deck (PN 19166097). Tall-deck versions (delivered with a 9.700-inch semi-finished deck) require spacers for intake manifolds, because the heads are moved farther apart than with the standard, 9.240-inch deck height.
Production automotive (and some truck) LS engine blocks are aluminum and reasonably robust for moderate boost pressure. If your plans for the engine exceed the roughly 800-hp range, a high-performance cylinder block is recommended. Although the strength of the block is crucial, the more important factor is the capacity for greater cylinder head clamping through the use of six head bolts per cylinder. Production blocks (including the supercharged LS9) use only four bolts per cylinder; although the LS9 uses larger, 11-mm head bolts versus other LS engines’ 10-mm head bolts.
There are high-performance LS blocks on the market, including the following.
GM Performance Parts LSX Block
Introduced in 2007, the LSX block is designed to support extreme highperformance combinations, especially high-boost engines. GM Performance Parts claims the block can support turbocharged engines making more than 2,000 hp and more than 20 pounds of boost. This is due largely to the provision for six bolts per cylinder.
The LSX block has a siamese-bore design, with 3.99-inch bores that must be finished to 4.00 inches—with a 4.25-inch recommended maximum bore. The maximum stroke can reach 4.25 inches, but rotating-assembly interference on the cylinder must be taken into account for strokes greater than 4.125 inches. It is offered in a production-style standard deck height (delivered .020-inch taller for machining purposes); and a tall-deck version with a 9.70-inch height.
Most significantly, the LSX block includes two additional head-bolt locations per cylinder (for a total of six) that greatly enhance clamping strength to prevent head-gasket blowouts under high boost. Street/ strip engines with up to about 15 to 19 pounds of boost will likely survive with conventional, four-bolt blocks, but if the engine is projected to use 20 pounds (or more) of boost, a six-bolt block is highly recommended. A second-generation LSX block was introduced in 2009 that offered several design and machining improvements, but retains the original PN. A first generation block is shown here.
Another advantage of the LSX block’s iron makeup is its capacity for machining. Its thick, siamese-type cylinders can be bored to 4.250 inches while retaining a minimum of .200-inch wall thickness. And, with machining, the standard-deck version can accept a 4.250-inch stroke, while the tall-deck version can take a 4.500-inch stroke. Proper machining, including the use of a deck plate as seen here, delivers a more accurate finished product that ensures greater cylinder sealing.
In the LSX block’s favor is its sturdy design, machining flexibility, six-bolt head-clamping strength, the availability of high-flow heads, and a very low retail price. Working against the LSX is the extra weight of an iron casting versus a production aluminum block.
GM Performance Parts C5R Race Block
When the only other choices for engine builders were production blocks, many turned to the unique C5R racing block that GM developed for its factory-backed Corvette racing team. It afforded a 427-ci displacement, and the block was considerably stronger than production blocks.
Although it makes a great foundation for moderately powered engines, it’s not optimal for higherboost combinations. That’s because the C5R block was designed to support 500 to 600 naturally aspirated horsepower. Most notably, it does not offer six-bolts-per-cylinder clamping. It is also expensive, although prices have come down in recent years.
The C5R is a wonderful piece of engine exotica, but the other cylinder blocks described in this section are better suited to supercharged and turbocharged applications.
The GM Performance Parts C5R cylinder block (PN 12480030) is very stiff and its specially machined, 356-T6M-alloy aluminum casting is X-rayed and “hipped,”which is a reference to the hot isostatic pressure process that pressurizes, heats, and cools the casting to virtually eliminate any chance for porosity. It is a time consuming procedure that contributes to the block’s nearly $6,000 list price. But for all its strength, the C5R was designed for about 750 to 900 naturally aspirated horsepower and, thus, does not include six-bolt head clamping as does the LSX block.
The C5R block was introduced prior to the LS7 engine, making it the only alternative for a larger-displacement, aluminum-block LS engine. But, availability of the LS7 block (PN 19213580) makes it a more economical choice for a lowerboost engine that doesn’t necessarily require six-bolt head clamping. It is typically offered at less than half the price of the C5R block.
Although the relatively economical LS7 cylinder block is robust enough for low- and mild-boost engines, there are definite product advantages to the more expensive C5R. The main bearing caps are a great example. On the left is the C5R and its racing-bred billet steel main caps and premium ARP studs and fasteners. On the right is the production-based LS7 block’s main caps and fasteners; they’re still strong pieces, but there’s a definite edge with the more expensive C5R.
Racing Head Service LS Race Block
New in 2009, the LS Race Block from Racing Head Service (RHS) is targeted at maximum-performance combinations, including forced induction. Like the LSX block, it features six-bolt head clamping, including a thick .750-inch deck. In fact, the head-bolt pattern is the same as on the GM Performance Parts LSX block, allowing great interchangeability with cylinder heads. Anything that fits the GM block fits the RHS Race Block. However, the LS Race Block is a lightweight, all-aluminum casting.
The RHS LS Race Block is the newest performance cylinder block on the market, and features the same six-bolt pattern as the GM LSX block. A raised-camshaft position (sized to accept a 60-mm camshaft) and oil galleries that are pushed outward enable a generous 4.600-inch stroke without rod-to-block interference. Various bore sizes are available, with the largest being 4.165 inch. Coupled with the maximum stroke, this aluminum, six-bolt block can offer more than 500 ci.
The block features a siamese bore design, but with pressed-in, spun-cast-iron cylinder liners. It is available with a minimum 4.125- inch-bore diameter and up to 4.165-inch bores. Both production and tall-deck 9.750-inch versions are available. RHS also touts the LS Race Block as “long-arm friendly,” with a raised camshaft centerline and outboard priority main oiling that enables greater rod clearance. A maximum stroke of 4.600 inches is achievable, delivering more than 500 ci with 4.165-inch bores Big displacement capability, interchangeability with GM LSX heads, and aluminum construction are the LS Race Block’s highlights. A comparatively high price is the only real negative.
World Products Warhawk LS7X Block
Long Island, New York–based World Products was the first to market with a high-performance, six-bolt, aftermarket LS cylinder block—beating even GM Performance Parts’ LSX block.
World Products’ Warhawk LS7X cylinder block is offered in four-bolt and six-bolt configurations, with the six-bolt versions (standard-deck 9.240 inches and tall-deck 9.800 inches) being the logical choice for forced induction. GM cylinder heads bolt right up to the standard four-bolt locations, but the six-bolt configuration is exclusive to World Products. This means using the World’s own six-bolt LS cylinder heads, which use 7/16-inch studs in all positions. The iron cylinder liners can be machined to 4.155 inches that, with the tall deck block’s 4.500-inch stroke capability, enables a 488-ci (8.0-liter) displacement. On the bottom side are billet-steel main caps that used ARP 200,000-psi main studs.
This is what the World Products Warhawk block looks like with the deck sliced off for an inside inspection. Note the cylinders and head-bolt holes are completely surrounded by a very generous water jacket, yet there is very thick material surrounding the cylinders. The extra-thick material adds strength to the cylinder areas, while also serving as a better insulator as the block warms and cools.
As with RHS’ Race Block, World’s Warhawk LS7X block is a lightweight aluminum casting. It offers six-bolt head clamping and is available in standard- and tall-deck (9.800-inch) versions. Billet steel main caps are standard and it weighs only about 135 pounds with the main caps installed. Additional details include:
- Range of cylinder bore diameters, from 3.990 inches to 4.115 inches
- Tall-deck version accommodates up to 4.500-inch stroke; standard deck takes up to 4.00-inch stroke, for a maximum displacement of more than 454 ci
- Priority main oiling; the oil circulates to the crankshaft first and the top of the engine is at the end of the oil circuit
- Cast-in provisions for standard small-block Chevy engine mounts, meaning a Warhawk based engine is more easily installed in an older GM car
- Provisions for an external oil gallery that enables greater displacement capability
- O-ring seals on the cylinder liners that prevent hot oil from squeezing between the block and liners, and heating the cylinders
The biggest detractor of the Warhawk block is that its six-bolt head pattern is unique. It is not the same as on the GM LSX or the RHS LS Race Block, meaning the only six-bolt cylinder head options come from World Products. That’s not necessarily a bad thing, as World’s heads offer tremendous flow attributes, but they’re the only six-bolt choices for the block.
Katech Re-Sleeved LS2 Cylinder Block
Katech Performance offers a modified version of the GM LS2 (6.0- liter) aluminum cylinder block. The company removes the stock, 4.000- inch iron cylinder liners and replaces them with larger, 4.125-inch-diameter bores that are also machined at the bottom to accept a 4.000-inchstroke crankshaft. This enables a final displacement of just about 428 ci, or 7.0 liters. The bores can be honed out to 4.130 inches, too, for a 429-ci maximum displacement.
This is GM’s standard 6.0-liter LS2 cylinder block, which is adequate for lowand moderate-boost engines that see primarily street and limited strip duty. Using a production-based, four-bolt block means keeping boost below 20 pounds. Production blocks are very affordable over the counter from GM dealers, and used cores are becoming less expensive as more show up at salvage yards. An even cheaper alternative (if you don’t mind the weight penalty) is an iron LS block from a truck, such as the Silverado or Suburban.
Katech offers the enlarged cylinder block with standard or billet steel main bearing caps. And while the larger displacement of the lightweight aluminum block is desirable, it retains the four-bolts-per-cylinder head-clamping pattern. That means an engine built with this block should be aimed at 1,000 hp or less and/or limited to less than 20 pounds of boost (assuming an allforged rotating assembly).
If building a larger-displacement engine isn’t a main priority for your project, all of GM’s productionbased, four-bolt LS cylinder blocks provide adequate strength for lowand moderate-boost engines.
Rotating Assembly
A forged crankshaft, forged rods, and forged pistons should be the ingredients that comprise the rotating assembly, but there are other factors to consider.
Crankshaft
Assuming a new engine build uses a forged-steel crankshaft, it’s important to understand that not all forged crankshafts are created equally. From the factory, only the LS7, LSA, and LS9 engines include a forged crankshaft; all other LS production crankshafts are cast iron.
It is possible to use the LS9 forged crankshaft in other LS engines and brand-new assemblies, but it has a considerably longer snout to support the dry-sump oiling system’s larger, gerotor-type oil pump, as well as a unique flywheel bolt pattern. It is possible to modify the crankshaft to work with other oil pumps and front-engine accessory drive systems, but it is easier and less expensive to spec a forged crankshaft from one of the well-known performance crankshaft manufacturers, such as Callies or Eagle.
But even under the banner of “forged steel,” there are different levels of forgings, based on the materials incorporated with the steel to enhance hardness and durability. The most common forgings used in performance engines are 4130 and 4340. Here’s what those numbers mean:
- The “4” refers to a steel alloy that is mixed with molybdenum for greater overall strength—the more “moly,” the tougher the crankshaft.
- The “1” and “3” numbers refer to other materials mixed in the alloy; the “1” indicates a steel alloy with chromium added, while the “3” in 4340 indicates nickel and chromium are part of the steel alloy, for even greater strength.
- The “30” and “40” numbers refer to the percentage of carbon added to enhance hardness; “30” refers to approximately 30-percent content and “40” indicates an approximate 40-percent content.
While both 4130 and 4340 forged-steel crankshafts are superior to standard cast-iron crankshafts, the 4340 forging is stronger than the 4130 because of its nickel content and higher percentage of carbon. Of course, that greater strength comes with a higher purchase price, but for racing applications it’s worth the investment. A street/strip engine does just fine with a properly prepared 4130 crankshaft.
Proper heat-treating can significantly strengthen the crankshaft, while crankshafts used in engines designed primarily for racing should also be shot-peened for maximum strength. Some builders also have the stress risers on the rod throws removed to improve performance and longevity.
A forged-steel crankshaft is a must for forced-induction engines in order to build strength into the engine assembly. They’re capable of supporting tremendous power levels of up to 1,500 hp or more. Billet-steel crankshafts are also available at a greater cost, but there is conflicting opinions on whether they deliver greater strength than a forged-steel crank made of the same material. As the name implies, billet-steel cranks are cut from a single piece of stock, while a forged crank is “pounded” into shape.
A 4340-alloy steel forging is the strongest available for crankshafts, while traditional heat-treating methods such as nitriding and induction hardening can enhance strength. With nitriding, the crankshaft is placed in an oven and ionized nitrogen is vacuum deposited on its surfaces; a process that can double the surface hardness. Induction hardening, where the sections of the crankshaft are subjected to a magnetic field for intense heating and cooling, can produce a harder surface, but only where the process was applied. Nitriding treats the entire crankshaft at once. Another common “trick” is knife edging the crank’s counterweights to reduce windage, but that has generally proven to be counter-effective to performance. The most efficient design is a rounded edge, as seen on the crankshaft being installed in this LSX block.
The crankshafts of production LS engines have a press-fit damper, and the only one with a crankshaft keyway to prevent slippage is the forged crank of the LS9. When building an engine for supercharged or turbocharged performance, a keyway is a must and you should spend the time and few extra dollars to have one machined into the nose of the crankshaft. Shown here is a “budget” supercharged engine build using a LS3’s cast crankshaft with a keyway cut into it.
When it comes to installing the crankshaft and main bearing caps, the cap fasteners should be the best you can afford. Rather than using the production-style combination of studs and bolts, all of the main caps should be secured with studs and nuts for more accurate fastening and repeatable removal and installation on an engine that will see moderate to frequent teardowns. ARP’s 200,000- psi studs and 12-point bolts (seen here) are the best on the market and should be highly considered.
Most of the fasteners on the LS engine feature torque-to-yield specifications. This means rather than a conventional foot-pound or inch-pound torque rating, the fasteners are final-tightened to a specific torque angle, such as 40 degrees or 60 degrees. So, a standard torque wrench is not enough and you will need to complement it with a torque angle wrench or a modern combination torque wrench (seen here) that includes pound readouts (and Newton meters) as well as angle degrees.
This is Thomson Automotive’s LS main bearing removal tool, and it’s pretty cool. The tight tolerance of the main bearing caps fitted to the longskirt LS-style cylinder block makes their removal difficult and time consuming. Thomson Automotive has come up with a simple, yet ingenious solution: a tool that hooks beneath the caps and uses the leverage of a pair of aluminum handles to yank the caps quickly and smoothly out of the block. Anyone who has struggled with removing LS main caps will appreciate this simple, but very useful tool.
To optimize lubrication, the engine may benefit from slots machined in the crankshaft journals that direct oil at higher RPM. Some racing-engine builders also useful groove bearings to ensure maximum oiling for the rods. Avoid cross drilling the crankshaft, however. While it was a common procedure years ago, most professional builders no longer believe it is effective. In fact, it may do more harm than good in the long run.
Reluctor Wheel
The crank-triggered ignition system of the LS engine requires a “reluctor” wheel (also known as a “tone” wheel) mounted on the crankshaft. It’s a toothed wheel that helps determine crankshaft position to ensure spark-timing accuracy. Early LS production engines came with a 24X (24 tooth), while later engines—including all those equipped with electronic throttle control—used a 58X (58-tooth) wheel.
Generally speaking, either wheel can be used on a custom engine build, but selection depends primarily on the engine controller to be used. The more common, later-style GM E38 and E67 controllers support the 58X wheel and electronic throttle control, while earlier LS1A and LS1B controllers support the 24X wheel. The 58X wheel can be used with earlier LS engines and later controllers, but revisions to the camshaft-position sensor requires an LS2/LS3 front cover on LS1/LS6 and some truck engines.
For example, a 24X wheel should be used if you plan to retain the original engine controller on an engine built for a 2002 Trans Am that was originally equipped with the LS1 engine. If, however, you plan to install an LS7 engine and supercharger, the LS7’s 58X wheel must be changed to a 24X wheel if the stock LS1 controller is to be used. Additionally, Lingenfelter Performance Engineering offers a conversion module that allows the 58X wheel to be used with earlier controllers, without the need for sensor or other wiring changes.
Aftermarket, standalone control systems, such as those from F.A.S.T. and ACCEL-DFI, are compatible with either the 24X or 58X wheel.
Pistons
The two most important factors for pistons in a forced-induction engine are cylinder pressure and strength. Simply stated, the castaluminum pistons of most production LS engines (only the supercharged LS9 comes with forged pistons) are adequate for low-boost, bolt-on power adders, but builders seeking higher power need stronger, forged-aluminum pistons that deliver a lower static compression ratio.
Although commendably lightweight and durable in naturally aspirated applications, the factory cast pistons’ high silicon content makes it rather brittle when compared with a forged-aluminum piston. That brittleness doesn’t stand up well to the excessive pressure generated by the blower or turbo; and it is especially susceptible to damage if detonation occurs.
Forged pistons are manufactured through a process that forms the part by essentially pounding it into shape rather than the poured metal of a cast piston. They are still comprised of alloys, but the manufacturing process brings greater material density and eliminates the chance for porosity, which greatly enhances strength. They’re also more ductile—the opposite of a casting’s brittleness—and they typically resist heat better than cast pistons. The best forgedaluminum pistons suitable for boost have less than 1-percent silicon content. (Production pistons are referred to as hypereutectic because of silicon content greater than 12 percent.)
At a glance, here’s how to tell the different GM reluctor wheels apart. The 24X wheel (left) is found on LS1, LS6, and other engines through about 2007, while the 58X wheel (right) is used on later engines, although there is some modelyear overlap between the reluctor wheel types on LS2 engines. The 2005 Corvette and most 2005 Pontiac GTOs use the LS2 with a 24X wheel. Generally speaking, the 58X wheel is used with Gen IV LS engines that moved the camshaft position sensor from the top rear of the engine block to the front of the engine, near the timing gear. In a custom project that will use a standalone control system, there’s not a significant reason to use one type of wheel over the other, but when building an engine for a primarily street-driven vehicle that was originally equipped with an LS engine and retains the original controller, it is best to use the original reluctor wheel design to ensure controller compatibility.
Lingenfelter’s TRG-001 conversion module is designed to enable later LS engines with the 58X reluctor wheel be used in vehicles with the earlier, 24X-based control systems without sensor or wiring changes. It plugs into the wiring harness and original sensors, although some early Gen IV engines may need a jumper harness to extend the camshaft position sensor wiring.
Here is a common, forged-aluminum piston design for LS engines. The flattop design, however, is not conducive to optimal forced-induction performance, as it would likely deliver too high of a compression ratio. Excessive compression can lead to enginedamaging detonation under boost.
Here’s a look at the production, forged-aluminum LS9 piston. Note that it is dished to minimize compression ratio, but that there is also a slight dome within the dish. It helps reflect the incoming air/fuel charge back toward the spark plug for greater combustion efficiency. It also has a unique ring pack that uses relatively thick top and secondary rings and a very thin, minimal bottom oil-control ring. The specialized machining process of the block enables tighter tolerances that, in turn, allows for the thinner, lowerfriction oil-control ring. Also note the friction-reducing Teflon coating on the skirt. In short, the LS9 piston is all about low friction and high rpm. It would make a good choice for low- to moderate-boost engines projected to make about 750 to 800 hp. (Photo courtesy General Motors)
A better piston selection for a supercharged or turbocharged street/strip LS engine is seen here in the “D”- shaped dish that provides a large, relatively efficient quench area. Quench is described as the squishing effect on the air charge as the piston reaches top dead center. The shape of the piston’s dish helps squeeze air through the combustion chamber in a manner that generally helps even out the temperature throughout the chamber and reduces the chance for detonation. The depth of the dish affects the compression ratio.
The side profile of a piston shows the crown height (defined as the space between the top ring land and the top of the piston). The minimum crown height for a forced-induction LS engine should be .200 inch; .300 inch is optimal. Production LS pistons don’t have such a thick crown, which (in addition to their cast construction) is why they’re not great in high-boost supercharged or turbocharged applications.
Generally, there are two grades of high-performance forged-aluminum pistons: 4032 and 2618. The 4032 forgings (which contain a small amount of silicon) are less expensive, but not as strong as silicon-free 2618- forged pistons. If there’s a trade-off with forged pistons, particularly 2618 forgings, it is increased coldstart engine noise due to thermal expansion. The silicon in hypereutectic pistons minimizes the piston’s expansion when the engine warms up, allowing for a much tighter piston-to-cylinder-wall tolerance, but the low silicon content of forged pistons means they “grow” more in the cylinder bore. Consequently, forged pistons need greater pistonto-wall clearance, with 2618 pistons needing the most.
In general, a 4032-forged piston needs approximately .0025- to .0035- inch piston-to-wall clearance, while 2618 pistons need about .0035- to .0045-inch clearance. That extra clearance means forged pistons typically generate an unsettling knocking noise known as piston slap when the engine is cold. The noise goes away as the cylinders and pistons.
heat up, causing the pistons to grow and fill up the space. (If the noise doesn’t abate after the engine warms up, it may indicate an incorrect engine assembly or other, more serious engine problems.)
Other attributes that contribute to a stronger “blower piston” include reinforced pin bosses (the areas on either side of the piston skirt where the pin slides in) and a thick piston crown. That’s the area between the top ring and the top of the piston. A thicker crown better withstands the punishment of detonation, as well as the generally hotter temperature and cylinder pressure that come with a highly boosted engine.
Besides selecting a forgedaluminum design, the pistons for a supercharged or turbocharged engine should be targeted to deliver a compression ratio between 8.5:1 and 9.5:1. This typically means using a D-shaped head with a dish (also known as an inverted dome) or strictly a dished piston and matching it carefully with the projected combustion chamber volume. Most LS production engines came from the factory with relatively high compression ratios, including greater than 10.25:1 (the LS7 engine has 11.0:1 compression). That’s too much compression for a forced-induction engine, making it difficult to prevent detonation.
One more thing: Along with strong, forged pistons, you should also employ heavy-duty piston wrist pins, even at the expense of adding weight to the assembly. As mentioned earlier, the overall weight of a forced-induction engine or its rotating assembly should be secondary to ensuring it is robust enough to withstand the pressure generated by the turbocharger or supercharger. To that end, heavier-yet-stronger wrist pins that are either larger in diameter, or have a thicker wall than those typically used in a naturally aspirated engine, should be considered.
Piston manufacturers such as JE Pistons and Diamond offer a variety of forged applications for LS engines and have excellent technical advisors to guide the builder into selecting the most appropriate parts.
Ceramic-Coated Pistons
On engines designed for higher boost and higher power levels, the use of ceramic-coated pistons is an effective way to combat excessive cylinder and combustion heat, while also reducing friction. Most piston manufacturers and companies with bearings for high-performance and racing engines offer parts with ceramic coatings. The coatings are generally based on Swain Tech products.
Ceramic-coated pistons can minimize both heat absorption and friction, but the builder must be extremely careful to ensure the coating is applied by a knowledgeable, experienced vendor or by the manufacturer itself. Poorly applied coating material or an incorrectly prepped piston can result in the very hard coating flaking or peeling off in the cylinder. This can cause catastrophic damage, as the coating will typically score, scratch, or gouge the cylinder walls—effectively ruining the cylinder block. This risk is typically not worth it on street/strip engines producing less than 1,000 hp and/or less than 15 or so pounds of boost. Use coated pistons on highpower racing engines where engine temperature will be greater.
On a piston, a coating on top reduces the heat absorbed by the piston, helping prevent burning or other damage under high-boost and leaner-fuel conditions. A coating on the skirts of the piston reduces heatbuilding friction and the same goes for coated main bearings. These coated parts come at a premium cost over non-coated components, but the hedge against the damage caused by excessive heat makes them wise investments.
Some builders use coated main bearings, too, but this is more of a preventative measure against the possibility of oil starvation, rather than a performance enhancement.
Piston Ring and Ring Pack
Piston rings service the vital job of sealing the cylinders to prevent combustion gases from entering the crankcase, while also controlling oil on the cylinder walls and stabilizing the pistons within the bores. Under the high pressure of supercharger or turbocharger boost, those jobs are all the more important, as maintaining cylinder pressure is essential to performance.
Piston manufacturers that offer “blower” pistons for forced-induction engines generally optimize the ringpack location to provide a generous crown for greater overall strength. But LS pistons nevertheless have a ring pack that is located closer to the crown than, say, old-school smallblock engines. The rings are typically thinner than previous-generation engines, but bring increased stability with reduced friction.
With the higher ring pack and pressure from forced induction, LS piston rings are subjected to significant heat. For the most part, that means using the strongest, most heat resistant rings you can afford. That typically means the top ring is molycoated or similar. Ductile iron has long been the mainstay of rings, but steel is used increasingly for its strength and durability.
Coated-steel piston rings or nitrided-steel wire rings are the strongest and most resistant to the heat that comes with forced induction (with temperatures that can exceed 600 degrees F). Because blow-by is a greater concern with turbocharging and supercharging, a tight ring end gap is necessary, but like other engine components, piston rings grow as the engine heats. That means the end gap is wider when the engine is cold and becomes narrower as the engine warms. Too tight of an end gap when the engine is cold can force the ring ends together with excessive pressure when the boosted engine generates greater heat, leading to failure. A general rule of thumb for forced-induction engines is a top ring gap of .006 inch for every inch of bore diameter. That means a 4.125-inch bore should have a top ring end gap of .025 inch. Consult the ring manufacturer in order to select the best parts, specifying the engine’s intended duty, operating range, and approximate power and boost levels.
Generally speaking, when it comes to ring end gaps, the tighter the gap, the better, as this generally maintains cylinder pressure and resists blow-by longer. Total Seal offers unique, two-piece gapless top and second rings that offer greater resistance to blow-by by preventing a conventional gap from opening between the ring ends.
While production engines’ ring sizes vary, most aftermarket LS pistons are manufactured to support 1.5-/1.5-/3.0-mm rings. Thinner rings can be used to reduce friction, but they are made from specialized material that makes them very expensive. They should only be used in a racing engine that will see repeated disassembly, as thinner rings wear out sooner and require more frequent replacement. Stick with thicker rings for street and street/strip combinations.
One more thing about piston rings: You should make sure they’re available for your desired bore size before ordering the pistons or having the cylinder block machined. Assembly plans go right off the tracks when the pistons arrive and there are no rings to fit them.
Gas Porting
The trick to gas porting involves drilling holes strategically in the piston to force the compression ring against the cylinder wall. The idea behind it is that this pressurized ring seal prevents the ring from fluttering at higher RPM, while extending the power curve.
Two types of gas porting are typically used: vertical and horizontal. Vertical gas ports are drilled from the piston deck into the top ring groove and behind the ring. This method is employed more by drag racers. Horizontal gas porting involves drilling holes through the bottom side of the top ring land, extending to the back wall of the ring groove. It is used more in circle track/road racing.
Generally, gas porting is best left to dedicated racing applications, where sustained performance at high RPM delivers the greatest benefit. Also, carbon builds up in the ports, so an engine that does primarily street duty (and does not get regular, between-race teardowns) quickly loses the advantage of gas porting when the ports clog. The pressure on the rings also significantly reduces the ring’s lifespan—another reason to avoid gas porting for street engines.
Connecting Rods
The higher the expected horsepower, the stronger and beefier the connecting rods need to be. Rod failures typically arise from high RPM strain and/or exhaust-stroke pressure. In general, greater horsepower increases the compressive force on the rods, while greater RPM increases tensile strain. These attributes are amplified considerably with forced induction
Most LS production engines use powdered-metal rods that, like their corresponding cast-aluminum pistons, are surprisingly robust in an unmodified engine. As mentioned earlier, factory engine components are designed to operate in a performance window within a few percent of the advertised horsepower and torque ratings. Consistently pushing beyond that range puts a strain on the internal components they weren’t designed for.
To withstand the strain under boost, high-performance connecting rods need to deliver greater compression strength and tensile strength. The typical upgrade is to a forgedsteel material, such as 4340 steel or 300M. Beyond the greater strength that comes with the denser material, these performance rods are typically thicker in key areas to enhance strength, too.
In most cases, builders choose between I-beam-style and H-beam style connecting rods. Each is known for delivering strength, but each delivers it slightly differently. The I-beam looks more like a conventional connecting rod, but is very thick through the middle, allowing it to handle great compressive loads. H-beam rods have a thin center section, but wide, flat outer sides that provide tremendous stiffness and resistance to bending.
Here’s an example of a gas-ported piston intended for drag racing, you can tell it by the the holes drilled through the piston head. Because those tiny holes can get clogged with carbon over even a relatively short period of operating time, gas porting is not an effective idea for street engines. The pressure created on the cylinder rings also wears them out much faster, requiring frequent replacement.
Typical example of I-beam (left) and H-beam (right) are shown here. In terms of strength, they are comparable when made of the same material, offering similar compression strength. The differences, then, are more subtle, and selecting one design over the other comes down to other engine assembly factors. Typically, an H-beam rod is lighter than an I-beam, but its big end is generally larger, too, which can mean a greater chance for cylinder block interference on a long-stroke combination. If block interference is not a concern, the extra rev capability enabled by lighter H-beam rods is preferred to offset the other higher-mass, heavy-duty rotating parts. Also, most professional builders insist on using a solid bushing on the small end, rather than the more common and cheaper split bushing.
Assuming all other attributes are equal, the I-beam and H-beam offer comparable compressive strength, but the thinner center portion of the Hbeam typically makes it lower in mass than an I-beam. The lighter H-beam design can make more of a difference with primarily street-driven vehicles, where more low-end power is desired.
Problems with performance rods can arise, however, with internal clearance within the cylinder block. Thick, racing-type I-beam rods on larger-stroke combinations (generally, engines greater than 427 ci) can interfere with the bottoms of the cylinders and other walls inside the block. Extreme care must be taken to gently rotate the rod/piston assembly to check for clearance problems. Notching the bottoms of the cylinders, making clearance for other areas within the block, and even machining the small and/or big ends of the rods may be required.
Performance connecting rods and rods used with stroker crankshafts may also cause interference issues with the windage tray. After the rotating assembly moves freely within the block, the windage tray should be installed and the engine carefully and slowly rotated to check for clearance problems. If any of the rods hit the windage tray, washers can be used as shims on the bolt studs. Two or three washers per stud are generally all that’s required to ensure adequate clearance.
4340 vs. 300M and Forged vs. Billet—and Aluminum
The common steel connecting rod forging is made from 4340 steel, which contains up to 2-percent nickel, along with smaller percentages of chromium, silicon, molybdenum, and manganese. It is an extremely durable material for connecting rods, but 300M alloy is gaining favor with many builders. It contains more silicon (approximately 1.5 percent) along with more moly and carbon.
Rods made from 300M can be more expensive, but they are generally stronger than a comparably sized 4340 rod, which enables the manufacturer to downsize the center section by up to 20 percent and still offer the strength of 4340 steel. In a supercharged/turbocharged engine that is already using a number of higher-mass components to reinforce overall strength, the investment in 300M rods can offset a significant source of rotating mass.
Another choice is to choose billetsteel over forged-steel connecting rods. As the name implies, billet rods are cut from a single piece of steel on a CNC machine. This is generally used for custom applications where a manufacturer may only make a few sets of a particular design that wouldn’t be cost effective to set up in a conventional forging operation.
A billet-steel rod can be stronger than a forged-steel rod, but only if the steel used is of higher quality than the 4340 or 300M recipes. Because the material does not have to be as malleable as the steel used in forging, it enables the manufacturer to use very strong steel.
As for forged-aluminum connecting rods, they offer very good strength and the obvious benefit of low mass—an attribute that helps offset the weight of heavy-duty piston and wrist pins. But aluminum rods have only about half the tensile strength of a steel rod and are much more susceptible to stretching and fatigue, so they are typically quite “chunky” in size in order to maintain their shape longer. This can cause cylinder-bore interference problems, requiring machining that could ultimately reduce overall strength. Aluminum rods are also considerably more expensive than forged-steel rods.
Aluminum rods are not recommended for street and street/strip engines. They are suitable for racing engines that will see frequent inspections and teardowns.
LS7/LS9 Titanium Connecting Rods
The titanium connecting rods of the LS7 and LS9 engines are strong and lightweight, enabling very quick RPM buildup, but not necessarily the best option when building a boostready engine. That’s because the rods are designed for the operating parameters of their respective factory engines.
This is a typical forgedaluminum connecting rod and at a glance it’s easy to see it is physically larger than a comparable forgedsteel rod. Aluminum rods tend to stretch the most during the cold-start period and during the compressive loads of varied RPM, which makes them particularly unsuitable for sustained use in a street vehicle. By some estimates, the lifespan of an aluminum rod is only 1/10 that of a forged-steel rod. They work best in a drag-racing engine where the engine is quickly brought up to high RPM and more or less left there during the run. There is an unquestionable impact on RPM capability with aluminum rods, but their fatigue rate makes them best left to racing-only combinations.
Note the extra-thick wrist pin being inserted into this piston/rod combination. In high-performance, forcedinduction engines, the wrist pin absorbs tremendous bending and radial pressure, so to shore up rotating assembly it should be large, robust, and made of a strong material, such as a 4130 forging for a street engine. For racing engines, perhaps the ultimate wrist pin is offered by Bill Miller Engineering in the form of its 9310 VAR (vacuum-arc-re-melt) steel pins.
A common issue with long-stroke engines and/or those using aluminum or even some forged H-beam-style rods is cylinder-block interference. The rotating assembly should be slowly and carefully turned after installation to check for potential interference, as seen here. This LSX-based engine will require additional machining to clear bulky forged-aluminum rods.
Looking upward at the bottom of the cylinder, here’s the machined block, showing the notched areas required for connecting-rod clearance. Although time consuming, as it must be accomplished on all of the cylinders, it is a relatively easy procedure to perform on an iron cylinder block. More care is required when dealing with the iron liners in an aluminum block. In fact, the machining requirements simply may not be possible on some aluminum blocks, forcing the use of different, lower-profile rods, a shorter-stroke length, or both.
Machining material off aluminum rods is another method of making them fit the tight confines of a bigstroke combination, but doing so can adversely affect strength and lower the lifespan of an already-stretchprone component that doesn’t have the tensile strength of steel.
Beefier connecting rods can squeeze the tolerance of the rod ends on the crankshaft journal. The rod side clearance should be between .00433 and .0200 inch.
All LS engines employ a windage tray. During assembly, it should be checked for interference with the crankshaft and rods. The process includes bolting down the tray after the rotating assembly has been installed and all the fasteners torqued to specification.
With the tray in place, slowly rotate the crankshaft and check for rod or crankshaft interference. Even if the assembly turns without hitting the tray, check for a too-close relationship that could lead to interference during engine operation.
Because the rods are validated to the strength requirements for their respective engines, higher boost and higher horsepower strain their compression-strength resistance. That’s not to say these rods are weak by any measure, but they’re simply not designed for use in, say, a 700-, 800-, or 1,000-hp forced-induction engine.
Sacrificing low-speed RPM capability for the assurance and longevity of a forged-steel rod is a worthy tradeoff.
Pre-Assembled Short-Block or “Crate Engine” Assembly
Several engine builders offer short-block and crate engine assemblies that are targeted at supercharged applications. Starting an engine with one of these can be a cost-effective and time-saving option, as you receive a pre-assembled portion of the engine with a correctly engineered engine base.
Generally speaking, a short-block assembly includes a cylinder block fitted with a crankshaft, rods, and pistons. Typically, there is no oil pump, oil pan, camshaft, or other components. An assembled longblock or crate engine generally adds cylinder heads, camshaft, oil pump, and perhaps oil pan, with other accessories and the induction system left up to you. More complete crate engines generally include an intake manifold and other accessories, such as a water pump.
This is an example of GM’s factory titanium connecting rods from the LS7 engine. The primary advantage to titanium rods comes in their lower mass (about 20-percent lighter) not superior strength over forged-steel rods. Titanium is very hard, but comparatively brittle when compared with steel, so the part must be designed for the RPM capability of the engine. Also, GM’s titanium rods use a split-style small-end bushing that some builders don’t like. Some companies, such as Katech Performance, offers the rods with replacement, solid bushings. (Photo courtesy General Motors)
If a windage-tray interference issue is discovered, stacking washers on the main cap studs that also secure the windage tray is the easiest method of curing the problem. Start with a single washer on each stud and re-check the clearance, adding washers until a satisfactory clearance is achieved.
Katech Performance’s Value 402 short-block kit is typical of blower-friendly assemblies. It uses all forged rotating parts, along with D-shaped, dished lowcompression pistons. The balancing and blueprinting steps performed during assembly make it a very good starting point for a home builder who wants to finish the engine with his heads and induction system. Similar short-blocks are available from World Products, using their Warhawk block, as well as other LS engine builders.
A good example of ready-to-go short-block assemblies are Katech Performance’s Value 402 (6.6-liter) and Value 427 (7.0-liter) kits. Each is built with a re-sleeved LS2 6.0-liter aluminum cylinder block, a premium 4340-forged-steel crankshaft, forged-aluminum pistons and forged H-beam connecting rods. The components are also balanced and blueprinted during assembly.
The rough cost of the Katech short-blocks is between $6,000 and $7,000, and while that may seem expensive compared to the budget small-block engines many enthusiasts grew up with, the aluminum block and other high-performance parts for the LS engine simply come at a higher price. Nonetheless, one of these short-block assemblies—or a similar assembly from another engine builder— makes a smart starting point for an engine combination.
Written by Barry Kluczyk and Posted with Permission of CarTechBooks
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